1. Field of the Invention
This invention relates generally to surgically-implantable prosthetic devices and, more specifically, to breast prostheses. The invention relates specifically to implantable breast prostheses with a low coefficient of friction between two or more interacting elastomeric shells in an aqueous fluid environment, without the addition of a lubricating agent to the fluid.
2. Description of Related Art
It has become a practice in the field of surgery to place a prosthetic implant in various areas of the body under any one of various conditions. In cases where cancerous, precancerous, or other abnormal or damaged tissue has been removed, the prosthetic implant is often used as a replacement for the removed tissue. Its purpose is to restore the original body contour. An implant of this character provides physical support for the surrounding body tissue. By filling any voids that are created by the removal of the body tissue, it preserves the normal outward appearance and feel of the body. Prosthetic devices have also been used to enhance or augment the appearance of body parts.
Breast prostheses have long been used for breast augmentation and for reconstructive surgery following a mastectomy. The prostheses are available in numerous sizes and shapes including teardrop, round, low profile, and high profile. Usually, breast prostheses are implanted via a small inframammary or peri-aerolar incision into a pocket dissected deep to the patient's own breast tissue in front of the pectoral muscle. In certain situations, the prosthesis may be placed behind the various chest muscles.
Some prosthetic devices have utilized a single shell or envelope, which is filled with a silicone gel, a saline solution, or other liquid, such as an oil or polymer. Other breast prosthetic devices have contained a combination of silicone gel and saline solution in separate compartments or lumens. Prior art silicone gel devices have tactile properties similar to normal tissue, but suffer from certain disadvantages. First, some silicone may bleed through the envelope and migrate out of the implant into the tissue, or into an adjacent saline filled compartment of the implant. Second, rupture of the envelope of a silicone gel implant is difficult for a patient to detect, and may require a Magnetic Resonance Imaging (MRI) scan for diagnosis.
Some breast prosthetic devices have utilized a single shell or envelope, which is filled with a saline solution. The prior art saline solution filled prosthetic devices suffer from certain disadvantages and lack the proper appearance and tactile properties due to several factors. First, the saline solution displaces too quickly to give the proper tactile properties. Second, the ease of displacement of the saline solution can create a “fluid wave” in the implant presenting an unnatural look of the prosthetic device. Third, when the saline solution displaces from one area of the implant, the lack of volume in that area may result in visible wrinkling of the shell. Fourth, the outer shell can fold upon itself, causing an area of wear (e.g., fold-flaw), leading to failure and deflation.
There are also breast prosthetic devices utilizing a single shell or envelope wherein the envelope contains baffle-forming material. The baffle-forming material fills at least a portion of the envelope, while the remainder of the envelope is filled with a fluid, such as saline solution. The baffle-forming material may or may not be attached to the envelope. The drawback to such prior art baffle-forming material is that such material may not match the single layer structure, geometry, proportions, etc., of the envelope, thereby resulting in wrinkling and folding of the implant due to the uncontrolled position of the baffle-forming material. Additionally, some of the prior art baffle material can bunch up in a portion of the implant and be felt through the shell, resulting in an unnatural feel to the implant. As described in U.S. Pat. No. 6,802,861, the position of the baffle-forming material within a breast implant can be controlled by a structure consisting of an inner shell defining a fluid-containing inner lumen surrounded by an outer shell defining a fluid-containing outer lumen and with one or more non-enclosing fitted shells in the outer lumen comprising the baffle-forming material. This arrangement of lumens, non-enclosing fitted shells, inner shell and outer shell decreases the displacement rate of the fluid, thereby improving the tactile characteristics and reducing the “fluid wave” effect compared to prior art single shell, saline filled devices. Furthermore, this non-enclosing fitted shell arrangement prevents wrinkling, folding or bunching together of the baffle material within the implant. Additionally, the outer shell is supported, feels smooth externally, and does not fold upon itself to create wear points. U.S. Pat. No. 6,802,861 teaches that a saline solution would be an appropriate choice for use as the fluid, however, other fluids may be utilized such as organic polymers or protein fluids. In addition, U.S. Pat. No. 6,802,861 teaches that lubricating agents may be added to the saline. While organic polymers, protein fluids and added lubricating agents may provide a consistent lubricating layer between the various interacting shells within the implant, in the event of rupture, they may cause an undesirable tissue response. Saline solution is the most desirable fluid to utilize to fill the implant, because in the event of rupture, saline does not cause a tissue response and is safely absorbed into the body tissue. However, saline may not provide a consistent lubricating layer between the various interacting shells within the implant, which may allow the shells in some areas of the implant to intermittently stick together and give the implant an undesirable tactile feel.
Typically, the shells of implantable breast prostheses are formed from solvent dispersions of high temperature vulcanizing (HTV) or room temperature vulcanizing (RTV) silicone. Aqueous fluids, such as saline solution, do not appreciably wet the surface of silicone, and are inviscid. Furthermore, a thin fluid film between two silicone surfaces can be easily displaced from an area, for example, when the shell surfaces are compressed together, as may occur when a breast implant with two or more shells is manipulated or palpated with fingertips. Displacement of the aqueous fluid from an area between the shells removes the aqueous fluid that was providing a consistent lubricating layer in that area, allowing the two silicone surfaces in that area to interact and “stick” as described above.
Silicone surfaces may stick together because they can have high coefficients of friction (static or kinetic), making it difficult for one silicone surface to slide while in contact with another silicone surface (ASTM standard D1894 is one means to measure the coefficients of friction of plastic film and sheeting). “Stiction,” a contraction of static friction, is also a term sometimes used to describe the interaction of such surfaces and their tendency to form cohesive or adhesive bonds, which cause them to stick together. This type of bonding can have a chemical basis (e.g., hydrogen bonding, Van der Waals forces, or electrostatic forces), or a mechanical basis (e.g., interlocking asperities), or a combination of both.
Various surface chemical modifications, applied coatings, lubricating fluids such as organic polymers and addition of lubricants to aqueous fluids such as saline have been proposed to lower the coefficient of friction and improve how two elastomeric surfaces interact and slide across each other (“slidability”), thereby reducing sticking and abrasion of the surfaces. See, for example, U.S. Pat. No. 5,736,251. Chemical modifications of the surface (e.g., plasma) and application of coatings to the surface can alter the surface of the silicone, changing how two surfaces interact. Lubricating fluids such as organic polymers and the addition of lubricants to aqueous fluids such as saline provide a physical layer separating the two silicone surfaces so that the surfaces do not directly interact and stick. U.S. Patent No. 4,731,081 describes the addition of a lubricant to the saline solution in a breast implant to increase slidability of the interior surfaces of the shell when folded upon itself.
Rather than the direct addition of a lubricant to the fluid in a lumen to separate the surfaces between interacting silicone shells of a breast implant, a lubricant can be indirectly added to the fluid in a lumen by diffusion through the shell from an adjacent silicone gel filled lumen. For example, dual shell/dual lumen breast implants have been marketed that are a combination of silicone gel filled and saline filled lumen. Considering an implant that has a silicone gel filled inner lumen and saline filled outer lumen, lubricating chemical species can diffuse through the inner lumen shell into the outer lumen, providing a layer of lubricity if contacted by the interior surface of the outer shell and/or the exterior surface of the inner shell. As an additional example, a dual shell/dual lumen implant with a silicone gel filled outer lumen and saline filled inner lumen can have lubricating chemical species diffuse through the inner lumen shell into the inner lumen, providing a layer of lubricity if contacted by the interior surface of the inner shell.
It is an object of the present invention to provide a surgically-implantable prosthetic device filled with only saline and/or other aqueous fluids and which has the appropriate tactile feel, appearance, and other characteristics found in a human breast. The present invention provides a low coefficient of friction and high slidability between two or more interacting elastomeric shells in an aqueous fluid environment without the addition of a lubricating agent to the fluid, thereby providing the implant with a natural tactile feel that is similar to human breast tissue.
Another object of this invention is to provide a low coefficient of friction (e.g., static and/or kinetic coefficient of friction) and high slidability between two or more interacting silicone shells in an aqueous fluid environment within a prosthetic device with two or more shells, by applying a surface texture on at least one of the two shells that interact to maintain a sufficient amount or volume of fluid in reservoirs at surfaces to provide a consistent lubricating layer between the two interacting shells.
Yet another object of this invention is to provide a consistent lubricating layer of aqueous fluid between two or more interacting shells within an implant with two or more shells, to reduce or prevent intermittent sticking together of the shells in one or more areas, thereby giving the implant a more desirable and natural tactile feel.